1. Field of the Invention
This invention relates to a method for controlling weft insertion in an air jet type loom that enables to minimize air consumption required for weft insertion.
2. Description of the Related Art
Air jet type looms are generally operated in such a manner that weft yarn is jetted out through a main nozzle together with air jetted out from a plurality of sub nozzles disposed downstream of the main nozzle in the running direction or path of weft yarn and is inserted to a shed of warp yarn.
Generally, sub nozzles are divided into a plurality of groups arrayed along the running path of weft yarn. Air is jetted out from each group of sub nozzles in a relay manner one after another from upstream side toward downstream side of the running direction of weft yarn by appropriately setting the timing and duration of jetting air with respect to each group of sub nozzles. Thus, weft yarn is securely inserted every time picking operation (weft insertion) is performed without loosening the weft yarn. In other words, each group of sub nozzles jets air in the weft inserting direction toward a tip end of the running weft yarn to help smooth running of the weft yarn.
Running properties of weft yarn are not constant in the length direction of weft yarn. In view of this, there is proposed a technique of properly regulating the air jet timing and air jet duration of sub nozzles depending on the running properties of weft yarn (e.g., see Japanese Unexamined Patent Publication No. 10-310951). Specifically, this publication proposes delaying the air jet end timing of the sub nozzles to thereby extend the air jet duration of the sub nozzles, and advancing the air jet start timing of sub nozzles in transient periods such as immediately after start-up of the loom or replacing time of a weft supplying body of supplying weft yarn when the running properties of weft yarn are lowered, considering a phenomenon that apparent or actual running properties of weft yarn are temporarily improved during these transient periods.
In the aforementioned prior art, however, since the air jet start timing of the sub nozzles is advanced merely during the transient periods, the prior art does not provide any contribution to improvement on running properties of weft yarn resulting from factors other than the aforementioned phenomenon seen in the transient periods. The prior art encountered problems such as loosening of weft yarn due to undesirable lowering of running speed of weft yarn and deterioration of quality of fabric resulting from such loosening of weft yarn. If the air jet start timing of the sub nozzles is set well in advance prior to a reference timing in an attempt to avoid such a drawback, air consumption is excessively large, which is uneconomical in the aspect of production cost of fabric.